Weight loss with absolutely no reason, unable to gain

Hi, for the past couple of years I've been always having some difficulty in gaining weight but at least it was stable. For the past couple of months something's changed and I started to lose weight fairly quickly. In a period of 4-5 months I went from 154 lb to 134.4 lb doing absolutely nothing but laying around.

I initially thought it was because of my diet (a ration of quinoa with chicken), then I changed it to a full plate of rice with beans, beef and potatoes for lunch and dinner (trust me, it is A LOT of rice). I been also having sort of lose stools, been taking fiber supplements daily, which kind of helps but I need to consume it daily in order to do any effect.

Went to a gastroenterologist and had a couple of exams done to test for diabetes, malabsorption, over/underactive thyroid, ESR, ultra-sensitive PCR, worms, colonoscopy, endoscopy, contrast x-ray for gastrointestinal transit, and the tests to diagnose pheochromocytoma (catecholamines, acid vanilmandelic, etc), HIV, Hep A, B, C. It all came back normal, and the doctor said my weight loss does not worry him at all, if it was cancer or anything like that it would accuse on the ESR and there would be more symptoms, like pain. So, he suggested that I was depressed and hypochondriac. I can tell you right now that I'm not depressive at all, I'm always happy and have tons of friends, oh yeah and I LOVE to live.

I simply cannot understand why I lost weight in the first place and why I am unable to gain it back. I see people on the street and they note how I lost weight and stuff.

The only confirmed medical condition that I do have are gallstones, and I am about to remove it, but I'm scared due to the lack of weight, and if I have to undergo a diet after surgery I might lose even more weight. The stone is 2cm in diameter and it produces no symptom aside from a really annoying indigestion (super bloated) and belching. Oh, just to make it clear, I do have loose stool.

Things I forgot to mention: I have very low LDL cholesterol (even though I shouldn't, I like to eat a lot of fat), and the CBC revealed a very normal and very healthy results. Vitamin screen also revealed normal results.

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